1. Field of the Invention
The present invention relates to a vacuum refining method for molten steel. More particularly, the present invention relates to a vacuum refining method for refining molten steel with a straight barrel type vacuum vessel having no vessel bottom.
2. Description of the Related Art
In a vacuum refining furnace, oxygen gas is blown onto molten steel to be refined by means of top-blowing. The objects of blowing oxygen gas by means of top-blowing are described as follows. The first object is "decarburization" in which oxygen gas is reacted with carbon contained in the molten steel when oxygen gas is blown. The second object is "Al heating" in which the temperature of molten steel is raised when Al added to molten steel is burned by oxygen gas blown onto the molten steel by means of top-blowing. The third object is "desulfurization" in which flux, such as lime, is added to molten steel together with carrier gas. The fourth object is "burner heating" in which oxygen gas and combustion improving gas of a hydrocarbon, such as LNG, are blown by means of top-blowing so as to heat a vacuum vessel and suppress the adhering metal.
Conventionally, DH is known as a vacuum refining furnace composed of a straight barrel type vacuum vessel and a dipping snorkel. However, in the case of DH, a vacuum vessel to circulate molten steel goes up and down, and no molten steel exists in the vacuum vessel when it is moved to the uppermost position. Accordingly, in the case of blowing oxygen gas by means of top-blow, oxygen gas directly collides with the bottom of the vacuum vessel. Therefore, refractory material of the vessel bottom is severely damaged by the colliding oxygen gas. For the above reason, a method of blowing oxygen gas from a top-blowing lance has not been adopted at all.
Although it is not a case of vacuum refining, a secondary refining furnace in which the top-blowing of oxygen gas is conducted with a straight barrel type dipping snorkel is described as "CAS-OB Method" in S1086 of vol. 71 of "Iron and Steel" published in 1985. The object of the above method is to raise the temperature of molten steel by burning Al. However, the following problems may be encountered according to the above method. In the above method, it is impossible to conduct pressure reduction processing. Accordingly, when it is necessary to conduct a very low carbon steel melting processing and a dehydrogenation processing together with "Al heating", it is necessary to provide another refining furnace, so that the equipment cost is increased. Since the operation is conducted under atmospheric pressure, molten steel can not be sufficiently agitated, and the heat transfer efficiency is low. In order to improve the heat transfer efficiency, it is necessary to extend the processing time.
In the decarburizing reaction treatment conducted for producing ultra low carbon steel by means of top blown oxygen in a region, the carbon concentration is not more than 0.1%. Since the carbon concentration is very low, oxygen gas which has been blown out by means of top-blowing temporarily generates an iron oxide on the surface of molten steel, and this iron oxide reacts with and is reduced by carbon contained in the molten steel. In order to facilitate the reducing reaction, it is necessary to raise the hot point so as to form an advantageous condition from the viewpoints of thermodynamics and reaction speed. Therefore, it is necessary to conduct a so called hard-blowing operation in which the top-blown oxygen is made to collide with the surface of molten steel at high jet intensity.
With regard to a molten steel refining method in which an RH type vacuum refining apparatus having a vessel bottom is used and a water-cooled type top-blowing lance inserted into a vacuum vessel from an upper portion blows out a jet stream of oxygen into the vacuum vessel for refining molten steel, an example is shown in Japanese Unexamined Patent Publication No. 2-54714. Therefore, this molten steel refining method is well known.
FIG. 8 is a schematic illustration showing a refining method of molten steel conducted by a conventional RH type vacuum degasifying apparatus. The operation will be explained below. There is provided a snorkel of up-leg 23 at the vessel bottom 22 of the vacuum vessel 21. Gas is blown into the vacuum vessel 21 from a lower end of the snorkel of up-leg 23, so that the molten steel 24 can be sucked up from a ladle 25 to the vacuum vessel 21. In the vacuum vessel 21, an oxygen jet 27 is blown out from a top-blowing lance 26 to the surface of the molten steel 24. In this way, the molten steel 24 is subjected to decarburizing processing and Al heating, and the thus processed molten steel 24 is returned to the ladle 25 via a snorkel of down-leg 28. When the molten steel 24 is circulated between the ladle 25 and the vacuum vessel 21 in this way, it is continuously processed.
However, when oxygen is fed from the top-blowing lance 26 in the RH type vacuum refining apparatus described above, since the vacuum vessel 21 has a vessel bottom 22, the operation is restricted in various ways, and the following problems may be encountered.
In the RH type vacuum refining apparatus, vacuum necessary for sucking up the molten steel 24 from the ladle 25 so as to make the molten steel 24 reach the vessel bottom 22 of the vacuum vessel 21 is usually not more than 200 Torr. In order to circulate the molten steel 24 after that, vacuum is further enhanced, and it becomes necessary to maintain a high vacuum of not more than 150 Torr. Further, when oxygen gas is blown out from the top-blowing lance 26 in a reduced pressure condition, it is necessary to maintain a high vacuum condition. Unless a high vacuum condition is maintained, an oxygen jet 27 collides with the vessel bottom 22, and the refractory material at the vessel bottom is damaged because the molten steel depth T is small. Accordingly, in the case of conducting the hard blowing operation, the following restrictions must be observed. In order to keep the depth L of a cavity 29, for example, it is necessary to keep a very high vacuum of about 10 Torr so that the head of molten steel can be raised to maintain the depth T of molten steel on the vessel bottom 22 in the vacuum vessel 21.
In the case where oxygen is blown out from the top-blowing lance at a low degree of vacuum, a quantity of molten steel to be sucked is small, so that the depth T of molten steel in the vacuum vessel 21 is small. Therefore, for the same reason as that described above, the oxygen jet 27 collides with the vessel bottom 22, and the refractory material at the vessel bottom is damaged. Therefore, the depth L of the cavity formed by the oxygen jet 27 is restricted. As a result, it is impossible to conduct the hard-blowing operation, and it is necessary to conduct a so called soft-blowing operation in which the top-blown oxygen is made to collide with the surface of molten steel at low jet intensity.
Consequently, in the RH type vacuum refining apparatus, the following problems may be encountered. When oxygen gas is blown out in a reduced pressure, it is restricted as described above. Since it is impossible to conduct a hard-blowing operation in a low degree of vacuum at the beginning of the treatment, the reduction of iron oxide is delayed and the decarburizing reaction speed is lowered. In addition to that, the jet speed of the oxygen gas is low. Therefore, after the lance has been discharged, oxygen in the periphery of the jet reacts with CO gas in the atmosphere, so that CO.sub.2 is generated. That is, the post combustion is actively conducted, for example, at a rate of post combustion that is not less than 20%. Accordingly, the temperature in the vessel is unnecessarily raised and the refractory material of the vacuum vessel is damaged.
On the other hand, when a vacuum refining apparatus, which will be referred to as a straight barrel type vacuum refining apparatus hereinafter, is used for refining, in which a lower portion of the straight barrel type vacuum vessel having no bottom is dipped in the molten steel in the ladle, it is possible to blow out oxygen even in a low degree of vacuum because there is provided no vessel bottom. When oxygen is blown out by means of top-blowing in the above refining apparatus, it is necessary to maintain the vacuum refining apparatus in a low degree of vacuum in order to facilitate the decarburizing reaction. The reason is that it is difficult for iron oxide to flow out from the vacuum vessel in the case of an unnecessarily high degree of vacuum, so that the decarburizing efficiency is lowered. To the contrary, when the degree of vacuum is too low, the circulation of molten steel is deteriorated, and molten steel can not be sufficiently mixed. Accordingly, the decarburizing efficiency is lowered.
Examples in which stainless steel is refined by means of top-blowing in the above straight barrel type vacuum refining apparatus are disclosed in Japanese Unexamined Patent Publication No. 1-156416, No. 61-37912, No. 5-105936 and No. 6-228629. In the above examples, the carbon concentration at which decarburization starts is in a high carbon concentration range of not less than 0.2%. Further, in the above patent publications, there is no specific description about the oxygen blowing condition.
In the decarburizing reaction conducted at the aforementioned high carbon concentration, the top-blown oxygen directly reacts with carbon in the molten steel since the carbon concentration is high. In the above circumstances, no iron oxide is generated. Accordingly, even if converter slag exists in the vacuum refining apparatus, no problems are caused. Also, since the carbon concentration is sufficiently high, the agitating and mixing characteristic and the decarburizing efficiency are not affected. Accordingly, in this case, the higher the vacuum in the vacuum refining apparatus is, the more effectively the decarburization can be conducted. In the above well-known documents, Japanese Unexamined Patent Publication No. 5-105936 discloses an example in which the degree of vacuum is maintained at 200 Torr, and Japanese Unexamined Patent Publications No. 1-156416, No. 61-037912 and No. 6-228629 disclose examples in which the degree of vacuum is kept at 100 Torr or 50 Torr.
In the case where the carbon concentration is high, from the viewpoint of the principle of decarburization, the higher the degree of vacuum is, the more advantageous the effect that can be provided. However, in order to keep the vacuum refining apparatus in a high vacuum condition, the investment in plant and equipment is necessarily increased for the vacuum pump system because a large quantity of CO gas is produced, and further molten steel splashes violently in the process. Therefore, it is necessary to increase the height of the apparatus for the prevention of splash. As a result, the investment in plant and equipment is increased. For the above reasons, in the above examples, the degree of vacuum is maintained at 100 Torr or 50 Torr. In the above well known documents, it is described that refining is continued until the carbon concentration becomes 0.01 to 0.02%. However, metallurgical effects are not shown when the carbon concentration is restricted to a value lower than 0.1%.
However, as described later, in a high vacuum condition in which the degree of vacuum is higher than 105 Torr, it is difficult for slag particles in the molten steel to flow out from the vessel, so that the decarburizing oxygen efficiency is low. Therefore, in the case of a degree of vacuum lower than 195 Torr, the agitating energy is reduced, and the molten steel can not be agitated and mixed sufficiently. For this reason, the decarburizing efficiency is lowered.
Japanese Unexamined Patent Publication No. 7-179930 discloses an example in which plain carbon steel was refined under the condition that the degree of vacuum was maintained at 200 Torr and oxygen was blown by means of top-blowing so that the carbon concentration was in a range from 0.03% to 0.001%. In this case, the post combustion rate was not less than 78%, and the decarburizing oxygen efficiency was very low. The reason was that the cavity depth, which was found by calculation using the expression described later, was only 52 mm. That is, the oxygen gas collided with the molten steel in the manner of soft blowing. Also, it can be considered that the degree of vacuum was too low, so that the molten steel was not agitated and mixed sufficiently and the decarburizing efficiency was further deteriorated. Japanese Unexamined Patent Publication No. 6-116627 discloses a method in which the molten steel, the carbon concentration of which is 0.03 to 1.0%, is subjected to a top-blown oxygen, and the vacuum P is controlled in accordance with the equation of P (Torr)=a+980.times.%C! (a=170 to 370). The object of this method is nitrogen removal. Although there is no description about the decarburizing efficiency, the degree of vacuum is 199 to 399 Torr when the carbon concentration is 0.03% which is the lowest value. In the low degree of vacuum described above, the stirring energy is lowered. Therefore, the molten steel can not be stirring and mixed sufficiently, and the decarburizing efficiency is deteriorated. Further, there is no description about the manner of blowing of oxygen, which is an important factor to enhance the decarburizing efficiency, in the above patent publication. That is, there is no description of whether the hard blowing operation or the soft blowing operation is conducted.
Japanese Unexamined Patent Publication No. 6-116626 discloses a technique in which molten steel is refined in a degree of vacuum of 760 to 100 Torr while a mixing ratio of top blown oxygen gas and Ar gas is changed in accordance with the degree of vacuum. There is a description that the carbon concentration at the start of decarburization is 1.0 to 0.1%. This operation is mainly conducted at a high carbon concentration. Even in this case, there is no description about the manner of blowing of oxygen, which is an important factor to enhance the decarburizing efficiency, in the above patent publication. That is, there is no description of whether the hard blow operation or the soft blow operation is conducted. Further, there is no description about the effective decarburizing condition when pure oxygen gas is used.
In the prior art in which the straight barrel type vacuum refining apparatus is used, examples are shown in the case of a region in which the carbon concentration is high and also in the case in which the degree of vacuum is too low, wherein the decarburizing principles are quite different from each other. Concerning the oxygen blowing condition, it is only recognized that the soft blow operation is required in the example, and no technical investigation has been made into the appropriate oxygen blowing condition.
In the straight barrel type vacuum refining apparatus, the following operation is effective. Before blowing oxygen gas into the vacuum vessel for the purpose of decarburization, in order to raise the temperature of molten steel in the vacuum vessel of the refining apparatus, Al alloy is added to the molten steel. Top blown oxygen is fed onto the surface of the molten steel, so that Al is burned to raise the temperature of the molten steel. The aforementioned Al heating is a technique in which Al alloy is continuously added to the molten steel or Al alloy is added to the molten steel all at once, and during the above Al alloy adding operation, oxygen is top-blown to the molten metal, so that Al is oxidized and the temperature of molten steel is raised by the heat generated by the oxidization of Al. In this case, when carbon contained in the molten steel is oxidized, the amount of oxygen used for oxidizing Al is reduced. Therefore, it is not preferable to oxidize carbon contained in the molten steel. It is necessary to react the top-blown oxygen with Al at a high efficiency. Also, it is necessary to add the thus generated heat to the molten steel at a high efficiency. From the viewpoint of thermodynamics, carbon and Al are respectively oxidized as follows. When the partial pressure of CO is high, that is, when the vacuum is low, the oxidization of Al occurs prior to the oxidization of carbon. However, when the partial pressure of CO is low, that is, when the vacuum is high, the oxidization of carbon occurs prior to the oxidization of Al. Consequently, the appropriate degree of vacuum has not been known in the actual operation for the following reasons. Although a low vacuum is necessary for suppressing the oxidization of carbon, in a free surface region in which the reaction occurs, the temperature is raised by the reaction, and the partial pressure of CO is not same as the degree of vacuum.
Further, it is necessary to effectively discharge Al.sub.2 O.sub.3 produced in the reaction outside the vacuum vessel. The reason is described below. When a large amount of Al.sub.2 O.sub.3 is suspended on the surface of the vacuum vessel, since the heat conduction of Al.sub.2 O.sub.3, which is an oxide, is low, Al.sub.2 O.sub.3 becomes a resistance to heat transfer. Accordingly, the coefficient of heat transfer on the surface region of the vacuum vessel is deteriorated, so that heat transfer efficiency is lowered. In order to discharge slag from the vacuum vessel, it is necessary to keep the vacuum vessel in a low degree of vacuum. The reason why the vacuum vessel is kept in a low degree of vacuum condition is described as follows. When the vacuum vessel is kept in a high degree of vacuum, an interval between the lower end of the dipping portion and the surface of the molten steel in the vacuum vessel is increased, and slag particles in the molten steel are moved in a stream flowing downward. However, very few of the particles of slag arrive at the lower end of the dipping portion, and most slag particles are circulating in the vacuum vessel. The above slag flow rises to a bubble activating surface being carried by a rising stream. Therefore, an amount of Al.sub.2 O.sub.3 suspended in the surface region is accumulated, so that the heat transfer efficiency is lowered.
An effective means for discharging Al.sub.2 O.sub.3 from the straight barrel type vacuum refining apparatus has not been found.
In order to effectively transfer the generated heat to the entire molten steel, it is necessary that an amount of circulating molten steel is sufficiently large. In this case, the amount of circulating molten steel may be smaller than that in the case of blowing oxygen performed for the purpose of decarburization. The reason is that not only convection heat transmission conducted by a circulating molten steel flow but also conduction heat transmission caused by a difference in temperature contributes to the heat transfer. However, in the case where the degree of vacuum is too low, gas blown into the molten steel expands greatly when it rises to the surface. Accordingly, the stirring energy is reduced and the molten steel is not agitated and mixed sufficiently. As a result, the heat transfer efficiency is lowered. Therefore, it is necessary that the degree of vacuum is maintained at the most appropriate value.
It is described in Japanese Unexamined Patent Publication No. 58-9914 that desulfurization is conducted after the high vacuum treatment of decarburization or hydrogen removal in the refining method of molten steel performed at a reduced pressure. In the above patent publication, a method is disclosed in which powder for refining is blown onto molten steel in a reduced pressure at a sufficiently high speed so that the powder can get into the molten steel. According to the above method, a flow speed of gas to be blown to the molten steel must be not lower than Mach 1, that is, when the flow speed of gas is higher than Mach 1, the powder for refining can get into the molten steel sufficiently.
According to the above method, the flow speed of gas to be blown to the molten steel is very high as described above. Accordingly, the molten steel splashes, and a lance and refractory material in the vessel are damaged, and further the metal adheres to the inside of the vessel. In order to remove the adhering metal, it takes time and labor. In order to blow the gas at a high flow speed of not less than Mach 1, it is necessary to reduce the nozzle diameter of the lance. Therefore, when a refining agent is blown into the vacuum vessel by the top-blowing lance inserted into it, in addition to the usual oxygen blowing hole, it is necessary to form a new blowing hole exclusively used for blowing the refining agent, which causes a problem with respect to the apparatus. On the other hand, when the refining agent is blown by the oxygen blowing lance, it is necessary to feed a large amount of carrier gas to ensure the blowing speed. As a result, the temperature is lowered, and further the utility cost is increased.
Japanese Unexamined Patent Publications No. 5-287357 and No. 5-171253 disclose a method in which an RH type vacuum refining apparatus having a vessel bottom is used and powder used for refining is blown from a water-cooled top-blowing lance inserted into a vacuum vessel so as to refine molten steel.
In the above patent publications, the following are described. In order to enhance the powder trapping efficiency, it is preferable to conduct a hard blow operation. When the hard blow operation is conducted in an RH vacuum refining apparatus, it is necessary to prevent the oxygen jet from colliding with the vessel bottom. Therefore, when oxygen gas is blown into the vacuum vessel from the top-blowing lance, it is necessary to ensure a head of molten steel in accordance with the depth of a cavity formed on the molten steel surface. For this reason, when powder for refining is blown into the vacuum vessel, a high degree of vacuum of not more than 100 Torr must be maintained. However, when the vacuum vessel is maintained in a high degree of vacuum condition, the amount of powder which is exhausted with the exhaust gas is increased. As a result, the powder trapping efficiency with respect to molten steel is lowered, and the reaction efficiency is deteriorated. In order to enhance the powder trapping efficiency, the blowing speed must be increased.
Concerning the circulating speed of molten steel in the vessel or ladle of the conventional vacuum refining apparatus, the renewal speed of molten steel is not high, so that a high blowing speed is required. However, when a jet speed of carrier gas is increased for the purpose of increasing the blowing speed of powder used for refining, the amount of flowing gas is increased and also spitting is increased. Therefore, it is not preferable to increase the jet speed of carrier gas. As is conventionally known, the speed of powder is a half of the speed of carrier gas at most, and further it is reported that the depth of intrusion of powder is constant irrespective of an amount of flowing carrier gas. For the above reasons, it is not advantageous that the speed of carrier gas is increased.
An example in which a desulfurizing agent is blown to molten steel in a straight barrel type vacuum refining apparatus is disclosed in Japanese Unexamined Patent Publication No. 6-212241. However, in the above patent publication, there is no description about the vacuum and flow speed which are important factors to determined the efficiency.
As described above, there is no disclosure of the condition in which the desulfurizing agent is added to molten steel in the straight barrel type vacuum refining apparatus.
In the refining method of molten steel conducted in a reduced pressure, when the composition of molten steel is adjusted after the process of decarburization or the processing in a high degree of vacuum, the temperature in the vacuum vessel is raised to suppress the adhering metal. In order to accomplish the above object, the molten steel is subjected to burner heating by using a top-blowing lance, so that the temperature of molten steel can be raised.
In the above case, since the pressure in the vacuum vessel is reduced, the length of a combustion flame blown out from the top-blowing lance tends to extend. However, when the flame reaches the surface of molten steel, a combustion improver of hydrocarbon, which has not burned yet, reacts with the molten steel, so that the concentrations of carbon and hydrogen in the molten steel are increased, which causes a serious problem. In order to solve the above problem, the degree of vacuum may be lowered so as to shorten the length of the flame, or an interval between the lance and the molten steel surface may be increased. In the case of RH, in order to circulate the molten steel, the molten steel must be sucked up into the vacuum vessel. Therefore, it is impossible to reduce the degree of vacuum. Accordingly, only one method of increasing the lance height can be adopted. However, according to this method, an interval between the average flame region and the molten steel surface is increased. Therefore, the heat transfer efficiency is lowered.
With regard to the burner heating conducted in a straight barrel type vacuum refining apparatus, there is no specific disclosure.